While doing a comprehensive quality control visit last week, our Pure Energy Final Inspector noted that both the atmospheric draft water heater and atmospheric draft boiler failed these safety tests:

Both spilled fumes for longer than one minute (the spillage tests failed)

Both had inadequate draft pressure in the flues (the minimum draft pressure wasn't met)

In addition to failing those tests, the negative pressure in the combustion appliance zone (CAZ) with reference to the outdoors was greater than allowed (the CAZ exceeded the maximum CAZ depressurization limit allowed) during the baseline and under the worst case conditions set up.

Upon further investigation, the Final Inspector found that, not only were the typical mechanical ventilation appliances making the CAZ negative, but the attic fan was on as well.

The attic was being ventilated to the outdoors by a typical attic fan installed in the roof. The pressure caused by the operating fan is sucking air out of the attic... and the house, and the CAZ, causing the CAZ to be under too great a negative pressure. Since every CFM of air that the fan exhausts has to come from somewhere, this negative pressure causes some of the make-up air to come down the flues rather than from the passive attic vents. The water heater and the boiler could not vent the flue gasses properly, and the fumes actually were being vented to the inside of the house. This is dangerous and unhealthy.

The reason the fan could suck air from the CAZ is because the attic was not fully separated from the house and from the basement as is required by the program and also from BPI.

So, not only does a leaky pressure boundary allow heated air to leak into the attic; moist air to leak into the attic; hot summer air to leak into the house; polluted air to leak into the house... but it also can impact the CAZ and cause CO and other pollutant poisoning.

Conclusions:

Attic fans can suck air down flues and cause health and safety problems for people.

Closed windows (due to air conditioning operation), can keep mechanical ventilation make-up air from coming from the outside through windows, and make-up air must come from somewhere, and this might be down flues instead.

Attic fans sometimes run year round due to poorly set thermostats. Sometimes the fans are on standard switches that people forget to shut off in the winter when the boiler is running.

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I think people put too much emphasis on trying to keep the attic cool as thereason to use a power vent.

First off people should be encouraged to use is the correct solution and not to spendmoney on band aids.

If the attic floor is air sealed and there are copious amounts of insulation thenan attic fan is a waste of money. If is it is not air sealed and lacks adequateinsulation than the first dollar(s) should be spent there.

With an attic fan you can reduce the temps a couple of degrees but you have to keep in mind that the main method of heat transfer from the roof to the attic floor is via radiant heat. Dropping the temps a couple of degrees does not change that. I will add get the fiberglass insulation out of the attic.

It doesn’t matter how much intake vent space there is if you are moving enough air to lower the attic temp you will have a negative pressure in the attic. If the negative attic pressure is greater than the house pressure you will have air floor from the house to the attic. Of course if you have a perfect air barrier then you can pull air from the house. If you have a large opening like around a flue or plumbing vent you could be pulling a lot ofair from the house. The more air you suck out of the attic the greater the negative air pressure and the greater the potential for pulling air from the house.

I see no benefit from using them in the winter to control moisture. Again the attic floor should be sealed and bath fans vented to the exterior so that the interior does not become a source of moisture in the attic. The sun heating the attic will result in lower RH than the cooler exterior air.

If you have ice dams then fix the air leaks and low insulation levels that are the primary culprits. On top of that make sure you have adequate soffit vents and air chutes between the rafters.

Proper natural ventilation should work for most homes. That means using lots of vent chutes and keeping insulation out of soffits and from air flow. These should be starting points for any home.

Lastly attic vents are only needed so far as to prevent humidity levels and to moderate attic temps. Attic venting is not intended to keep attic temps in line with ambient outdoor air temps. Natural ventilation will work when designed and installed properly.

Hi Joshua, the temperature and humidity controls necessary to use an attic fan to reduce ice damming are a bit complex and unless a smart box has popped up somewhere, not available. Ice dams can form from heat loss into the attic or from the sun penetrating the snow and warming a roof from the outside, but both only form ice under a very narrow range of conditions, which can be altered with mechanical ventilation. But without proper controls one risks pulling in moist air under a cold roof deck and that could be a disaster. As you said, wrong application.

I have had customers telling me their roofer wants to add an attic fan to solve ice damning issues. Now I will guarantee that the roofer is not going to move insulation around and make the ceiling air tight before installing that 1100CFM fan. I am sure it is a safe bet that the majority of the attic ventilators are installed as a quick fix and are causing more harm than good.

Hi Tamasin, Great topic that needs even more attention. First apologies if some of this was mentioned, I did not re-read a second time.

At the risk being drawn and quartered, I like attic fans. Natural ventilation requires the proper amount of high and low venting to work at best poorly. Wind can help, but on a calm hot day it is the difference in weight of hot air vs the incoming warm air (cold would be better) that powers a natural air exchange. Contrary to the often stated premise that hot air rises to exit the upper vent, it is in fact the invading cooler (heavier) air from below that pushes the hot air out. The difference may seem like semantics, however, the invading cooler air principle will create a more positive pressure inside the attic.

Adding an attic fan, just like proper natural ventilation, does indeed require proper sizing and proper installation. Done right, the attic temperatures will remain lower throughout the day and recover more quickly as evening arrives. A hot attic left to cool by itself requires far too long, IMO, the natural way.

The real problem with attic fans is they fall into the quick fix category where home owners hope that this one effort will be their total solution. They are not. But when properly installed along with proper venting, air sealing, insulation, and controls, they can provide a better cooling solution for many attics.

Add to your caution concerning attic fans, the whole house fans and oversized kitchen fans, all of which can over power a CAZ.

Ed, just as stack effect in the winter is caused by heated air rising inside a home and leaking out through leaks in the attic and second floor; reverse or negative stack effect can happen in the summer due to cooled indoor air descending and leaking out through the basement and first floor. This draws warm air in from leaks in the attic and second floor. Reverse stack effect is typically not as strong as positive stack effect because the temperature difference is not as great in the summer as it is in the winter. I'm speaking from an upstate NY perspective, of course. Summer or winter, the treatment is the same; air-seal and insulate the attic.

If one were to add an attic fan, it overpowers any stack effect air exchange taking place between the attic and top floor, whether the stack effect is positive or negative.